1. Field of the Invention
The present invention relates to communication devices and, more specifically, to a communication device for transmitting input data including at least a biphase-encoded portion with the use of an analog signal.
2. Description of the Background Art
Conventionally, in order to transfer digital audio data between two devices, biphase encoding has been generally used. One example of biphase encoding is defined in a format of Sony/Philips Digital Interface (S/PDIF). In biphase encoding, as illustrated in FIG. 6, each bit of original data is represented by two logical values. After biphase encoding, a logical value of ‘1’ of the original data represents a state transition (0 to 1 or 1 to 0, for example) occurring at the midpoint of one bit period while a logical value of “0” thereof represents the absence of a state transition (0 to 0 or 1 to 1, for example) occurring at the midpoint. Furthermore, at the boundary between bits of the original data, the logical value is always reversed (that is, after biphase encoding, the logical value becomes 1 if the previous value is 0, and 0 if 1).
As such, biphase encoding always changes the logical value at the boundaries between the bits of the original data. Therefore, even in a case where the original data has successive 0s or 1s, the receiving device can easily reproduce a clock signal from the transfer data without requiring transmission of any additional clock signal.
In general, the data transferred between the devices includes the audio data which is biphase-encoded in the above-described manner and is added with a preamble of 8 bits (which corresponds to 4 bits of the original data before biphase encoding) for establishing synchronization of this audio data. Biphase encoding is not applied to the preamble, and the bit string of the preamble includes three or more successive 01s or 1s. By contrast, as described above, the biphase-encoded data never includes such three or more successive 0s or 1s because the logic value is always reversed at the boundaries between the bits of the original data. Therefore, by determining whether three successive 0s or 1s have been received, the receiving device can easily discriminate the audio data portion from the preamble portion. A plurality of preamble patterns are provided in advance, such as one indicative of the head of a block (B preamble), one indicative of the head of a sub-frame of the R channel (M preamble), and one indicative of the head of a sub-frame of the L channel (W preamble). FIG. 7 illustrates header patterns (B header, M header, and W header) used in S/PDIF. Since the code is always reversed at the boundary between the header portion and the data portion in an S/PDIF frame, each header has two patterns in accordance with the previous code.
One example of a communication protocol that has been used in recent years for achieving data transfer between vehicle-mounted devices through an on-board LAN is Media Oriented Systems Transport (MOST). In MOST, as illustrated in FIG. 8, data is transferred in units of frames (MOST frames), and the above-described biphase encoding is applied to the data portion.
S/PDIF and MOST are communication protocols optimized for data transmission by using a plastic optical fiber (POF), but a copper wire, such as a twisted pair cable or a coaxial cable, can also be used as a transmission medium. One advantage of using such a copper wire is easy handling.
Even having an advantage of not requiring transfer of a clock signal, however, data transfer using biphase encoding has a disadvantage of an increased transmission band in order to achieve a certain data transfer speed. For example, in MOST, in order to achieve an effective transfer speed of 25 Mbps, a data transfer speed of 50 Mbps is required. Therefore, if biphase-encoded transmission data output from a vehicle-mounted device is sent out as it is to the copper wire, an influence of electromagnetic radiation from the copper wire cannot be ignored even if the twisted pair cable, which less likely has an external influence, is used as the transmission medium.
One scheme to solve the above problems is to map the transmission data which is structured of biphase-encoded data at predetermined signal levels so that two bits of the transmission data is taken as one symbol for transmission (refer to International Publication No. 02/30075, for example).
FIG. 10 illustrates one example of a communication system using the above-mentioned scheme. In FIG. 10, input data (biphase-encoded serial data) 102 structured of frames is first supplied to a transmitting unit 10 of one device. This input data is converted from serial to parallel by an S/P converter 104 to become parallel data of two bits. An encoder 106 maps the data of two bits sequentially output from the S/P converter 104 at the predetermined signal levels so that the data of two bits are taken as one symbol.
The operation of the encoder 106 is described below with reference to FIGS. 11 through 13.
As illustrated in FIG. 11, the encoder 106 of the transmitting unit 10 in the device maps the data of two bits sequentially output from an S/P converter 104 at any one of eight signal levels so that the polarity is changed per symbol. The signal level at which the data of two bits is to be mapped depends on the immediately-preceding mapping result. In one specific example, the signal level at which the transmission data is to be mapped is determined with reference to a table shown in FIG. 12 based on the signal level of the immediately-preceding symbol and the transmission data to be mapped. Consequently, the processing results of the encoder 106 areas illustrated in FIG. 13, for example.
The processing results of the encoder 106 are supplied via a digital filter 108 to a D/A converter 110, which converts them to an analog signal. This analog signal is output via a low-pass filter 112 and a differential driver 114 to a twisted pair cable 20.
The analog signal transmitted via the twisted pair cable is supplied via a differential receiver 304 included in a receiving unit 30 of another device to an A/D converter 306. Based on the output of the A/D converter 306, a synchronization processing unit 308 regenerates clock signals. An output of the A/D converter 306 is supplied via a digital filter 310 to a decision processing unit 312. The decision processing unit 312 decides one of predetermined signal level as the signal level at which the output of the A/D converter 306 was assigned. Based on the decision result, the encoder 314 outputs serial data. This serial data is identical to the input data 102.
In this way, every two bits of the input data 102 are mapped as one symbol at one of the predetermined signal levels for transmission. With this, compared with a case where one bit is transmitted as one symbol, the symbol rate can be suppressed by half, thereby reducing electromagnetic radiation. Furthermore, as illustrated in FIG. 11, with this mapping being performed so that the polarity of the signal level is always reversed per symbol, the transmission signal always includes a frequency component which is half of a symbol frequency. Therefore, more reliable clock regeneration can be achieved at the receiving device.
However, studies by the inventors of the present invention found that the above conventional communication system has a disadvantage such that the degree of emission noise can be differed depending on the timing of serial-parallel conversion, which is described below.
As illustrated in FIG. 14, there are two types of timing for serial-parallel conversion of biphase-encoded data. That is, a first timing corresponds to boundaries between bits of data before biphase-encoding, whereas a second timing does not. In the communication system illustrated in FIG. 10, it is not determined in which timing the S/P converter 104 performs serial-parallel conversion, and therefore a probability of conversion in the first timing and a probability of conversion in the second timing are both ½.
Here, in FIG. 14, two bits of parallel data obtained through serial-parallel conversion in the first timing are one of four pairs of “00”, “01”, “10”, and “11”. When such parallel data is mapped in accordance with the rules shown in FIG. 12, the transmission waveform can be at one of the eight signal levels as illustrated in FIG. 15.
On the other hand, in FIG. 14, two bits of parallel data obtained through serial-parallel conversion in the second timing are either one of two pairs of “01” and “10”, because of the characteristics of biphase encoding. When such parallel data is mapped in accordance with the rules shown in FIG. 12, the transmission waveform becomes as illustrated in FIG. 16A or FIG. 16B, depending on the state.
Here, compared with the transmission waveforms illustrated in FIGS. 15 and 16A, the transmission waveform illustrated in FIG. 16B has a higher possibility such that the signal level is changed from +7 to −7 (or from −7 to +7). When the signal level is alternately changed between +7 and −7, noise from the twisted pair cable becomes maximum. Therefore, the transmission waveform as illustrated in FIG. 16B is not preferable compared with the transmission waveforms as illustrated in FIGS. 15 and 16A. Particularly, when data before biphase encoding has successive 0s, the parallel data obtained through serial-parallel conversion in the second timing has “01” and “10” that are alternately repeated and, when such parallel data is mapped in accordance with the rules shown in FIG. 12, the signal level is changed alternately between +7 and −7, which is not preferable as described above.